Recent studies in our laboratory have established for the first time that the major efferent outflow from the arm area of the primate motor cortex is derived not from its widely studied population of fast PT cells, but instead from: (a) a much larger population of very small PT cells, the majority of which send their axons to the contralateral cord; and (2) a separate population of small corticorubral neurons, that exert excitatory effects upon rubrospinal tract cells. Additional studies of the motor functions of these small cell systems are therefore badly needed. We are therefore conducting three sets of experiments, that have been designed to provide new information about the voluntary 'phasic' and steady or 'tonic' motor functions of the monkey's slow pyramidal tract and corticorubrospinal pathways. In the first set, microelectrode recording techniques are used to observe and compare the behavior of these small cells with that of fast PT neurons, in monkeys that have been trained to generate both transient and maintained steady outputs in forelimb muslce tension. These experiments are supplemented by observations of the behavior of fast and slow-twitch motor units in the muscles of interest, during performance of the same behavioral tasks. In the second set of experiments, additional information will be obtained about the functions of the rubrospinal pathway, by recording the activity of cells within the red nucleus in animals that have been trained to perform precisely the same set of motor tasks. Finally, a careful, quantitative electromyographic study will also be carried out, in order to determine the specific motor deficits that are produced in the performance of these tasks by surgical interruption of pyramidal and corticorubrospinal pathways. BIBLIOGRAPHIC REFERENCES: Humphrey, D.R. & Rietz, R.R. Cells of origin of corticorubral projections from the arm area of the primate motor cortex and their synaptic actions in the red nucleus. Brain Res. 110 (1976) 162-169. Humphrey, D.R., Corrie, W.S. and Rietz, R.R. Sizes, intracortical locations and properties of major neuronal output populations within the arm area of primate motor cortex. Neuroscience Abstracts 2 (1976) 750.